NEW YORK (AP) _ Philip Morris Cos. will add another cigarette to its pack of smokes next month when it unveils a new version of its No. 1 Marlboro brand, industry sources said Friday.

The new cigarette, dubbed Marlboro Medium, will contain lower tar and nicotine levels than the company's full-strength Marlboro, but levels higher than those contained in its Marlboro Lights version.

Marlboro Mediums are being pitched at the trade level as a ''new low-tar cigarette when you want more flavor,'' according to sources who spoke on condition of anonymity. The product is expected to compete against Camel and Winston, the main brands of rival R.J. Reynolds Tobacco Co.

Philip Morris USA spokesman Les Zuke refused to comment on the new Marlboro, saying it was company policy not to discuss ''any products we may be developing until they are ready to market.''

He added, however, that the company always is ''exploring the development of new products to meet the changing tastes and preferences of the adult smoker.''

Marlboro, the nation's No. 1 cigarette, commands 26 percent of the market. But the cigarette industry is fighting an uphill battle: The number of smokers contiues to decline. According to John Maxwell, an analyst with Wheat First Securities Inc. in Richmond, Va., unit sales of the Marlboro brand fell 2.4 percent between 1988 and 1990.

Among those who continue to smoke, many are seeking lower tar and nicotine brands.

Smokers also are opting for lower-priced cigarettes. The retail price of a low-cost brand can be as much as 50 cents a pack cheaper than full-price cigarettes like Marlboros. (A cigarette's price is determined by the tobacco as well as the amount and type of advertising used to promote it.)

The introduction of Marlboro Mediums, which sources said took three years to develop, adds yet another variation to a market packed with name brands and generic cigarettes.

Critics have surfaced already, charging Marlboro Mediums might be a confusing and unnecessary extension of Philip Morris's product line that could defeat the purpose of trying to keep smokers from switching to budget brands as well as attract those who purchase competing cigarettes.

''In the last month alone, I think we've had seven new brands from manufacturers,'' said Steven Levine, a vice president at Sanders Langsam Tobacco Co., a Queens, N.Y.-based distributor.

''Companies are adding so many different cigarettes to the market, it's flooding it so much,'' he said, though added that Marlboro ''is in a class by itself.''

Philip Morris also makes and markets Merit, Parliament, Virginia Slims, Benson & Hedges, Players, Cambridge, Bucks, Bristol and Alpine cigarettes.

Americans consume about 525 billion cigarettes each year valued at about $29 billion.

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